High data rates are achieved in optical telecommunication systems using dense wavelength division multiplexed (DWDM) signals. DWDM signals contain multiple channel signals each at a predefined channel wavelength. Typically, the channel signals are within a wavelength range defined by the flat gain region of erbium doped fiber amplifiers (EDFAs), a critical component of modern optical telecommunication systems. Performance of the EDFAs and other system components can be verified by monitoring signal-to-noise ratio (SNR) of each of the channel signals. One method of SNR monitoring involves measuring noise power within narrow wavelength regions between the predefined channel wavelengths and comparing the measured noise power to the power of each channel signal. Since the component channel signals within a DWDM signal may be spaced as closely as 25 GHz, measuring noise power between the channel signals requires highly selective filtering within the wavelength regions between the channel signals. An optical demultiplexing device, taught by Scobey in U.S. Pat. No. 5,583,683 is useful for monitoring power of the channel signals, but the device is not sufficiently selective to monitor noise power in the wavelength regions between closely-spaced channel signals.